In recent time, wind turbines have become an attractive way of producing electrical power. In order to do so, the power in the wind is captured by a set of blades (normally two or three) of a wind power plant. The wind captured by the blades causes a shaft connected to the blades to rotate. The shaft is connected to a rotor of a generator, which hence rotates at the same speed as the shaft, or at a multiple of the speed of the shaft in case the rotor is connected to the shaft via a gearbox. The generator then converts the mechanical power provided by the wind into electrical power for delivery to a power grid.
There are a number of control problems associated with wind turbine generators. For example, there is a need to control the output power of the wind power plant and also to make it less sensitive to different kinds of disturbances that may occur, such as resonances in the drive train of the wind turbine or disturbances on the power grid.
WO 2008/145128 discloses a wind turbine power controller which is connected to a wind turbine generator and a power grid. The power controller comprises two control loops referred to as an external power control loop and an internal current/torque control loop, the external power control loop being considerably slower than the internal current/torque control loop. The wind turbine in WO 2008/145128 further comprises resonant control means which modifies the power reference value provided to the external control loop. In this way, influences of resonances in the drive train of the wind turbine may be minimized.
Having a slow outer control loop may be favorable from the point of view that it will act as a low-pass filter for high frequency disturbances in the power feedback signal. However, having a slow outer control loop will also cause the wind turbine generator to have a slow response to changes in the power reference signal. Thus there is a need for a controller which may have a fast response to changes in the power reference signal regardless of the existence of a slow outer control loop.